Involvement of Protein Phosphorylation in Water Stress-induced Antioxidant Defense in Maize Leaves

Using pharmacological and biochemical approaches, the role of protein phosphorylation and the interrelationship between water stress-enhanced kinase activity, antioxidant enzyme activity, hydrogen peroxide （H202） accumulation and endogenous abscisic acid in maize （Zea mays L.） leaves were investigated. Water-stress upregulated the activities of total protein phosphorylation and Ca^2＋-dependent protein kinase, and the upregulation was blocked in abscisic aciddeficient vp5 mutant. Furthermore, pretreatments with a nicotinamide adenine dinucleotide phosphate oxidase inhibitor and a scavenger of H2O2 significantly reduced the increased activities of total protein kinase and Ca^2＋-dependent protein kinase in maize leaves exposed to water stress. Pretreatments with different protein kinase inhibitors also reduced the water stress-induced H2O2 production and the water stress-enhanced activities of antioxidant enzymes such as superoxide dismutase, catalase, ascorbate peroxidase and glutathione reductase. The data suggest that protein phosphorylation and H2O2 generation are required for water stress-induced antioxidant defense in maize leaves and that crosstalk between protein phosphorylation and H2O2 generation may occur.

A Novel Mitogen-Activated Protein Kinase Gene in Maize (Zea mays),ZmMPK3,is Involved in Response to Diverse Environmental Cues

In search for components of mitogen-activated protein kinase （MAPK） cascades in maize （Zea mays） involved in response to abscisic acid （ABA） stimulus, a novel MAPK gene, ZmMPK3, from ABA-treated maize leaves cDNA was isolated and characterized. The full length of the ZmMPK3 gene is 1 520 bp and encodes a 376 amino acid protein with a predicted molecular mass of 43.5 kD and a pl of 5.83. ZmMPK3 contains all 11 MAPK conserved subdomains and the phosphorylation motif TEY. Amino acid sequence alignment revealed that ZmMPK3 shared high identity with group-A MAPK in plants. A time course （30-360 min） experiment using a variety of signal molecules and stresses revealed that the transcripts level of ZmMPK3 accumulated markedly and rapidly when maize seedlings were subjected to exogenous signaling molecules： ABA, H202, jasmonic acid and salicylic acid, various abioUc stimuli such as cold, drought, ultraviolet light, salinity, heavy metal and mechanical wounding. Its transcription was also found to be tissue-specific regulated. Here, we show that ABA and H202 induced a significant increase in the ZmMPK3 activity using immunoprecipitation and in-gel kinase assay. Furthermore, the results showed that the ZmMPK3 protein is localized mainly to the nucleus. These results suggest that the ZmMPK3 may play an important role in response to environmental stresses.